The invention relates to a method and an apparatus for monitoring the operational readiness of an oxygen sensor located in the exhaust gas system of the engine and responsible for generating signals for a mixture controller of the engine.
It is known to provide fuel-air mixture controllers for internal combustion engines in order to obtain exhaust gases free from toxic components. These controllers receive their signals from an oxygen sensor located in the exhaust gas system of the engine. This type of control is usually superimposed on a preliminary, coarse, forward control which determines the basic fuel-air mixture. A prerequisite for the satisfactory functioning of such a control system is the correct operation of the transducer, i.e., the oxygen sensor. Oxygen sensors which work on the principle of ion conduction through a solid electrolyte respond to a difference in the partial pressure of oxygen and generate a voltage signal according to the partial pressure of oxygen within the exhaust gas. This voltage signal undergoes an abrupt shift when the oxygen content in the exhaust gas makes a transition from an excess amount to a shortage, i.e., when the air factor .lambda. crosses the value .lambda. = 1. A clear output signal is obtained, however, only when the oxygen sensor has reached a certain minimum operational temperature. When the sensor is too cold, the internal resistance of the sensor is too large to generate a sufficiently large output signal and, in particular, to provide a sufficiently clear potential shift. For this reason, when such sensors are normally heated to the operational temperature by the exhaust gases of the engine, it is necessary to provide a preliminary mixture adjustment which is supplanted by closed-loop mixture control based on the oxygen sensor output signal after the latter reaches its normal operational temperature.
In some operational states of the engine, the oxygen temperature may drop below its optimum value even when the engine is running. At any time, a defect in the sensor itself may cause suppression of the signal. If no precautions were taken to account for these malfunctions, they would lead to extremely erroneous adjustments of the fuel-air mixture. Furthermore, the mixture adjustment provided during the warm-up phase of the engine is not usually optimized with respect to the exhaust gas conditions so that a monitoring system for checking the operational readiness of the oxygen sensor is required.
In a known monitoring system of this kind, the alternating voltage signals coming from the oxygen sensor, the frequency of which depends on the manner of operation of the engine, are fed to a threshold switch that holds a switch in a first position as long as the period of sensor alternations is shorter than the time constant of the switch. In the opposite case, the switch is brought into its second position after the expiration of the time constant. The switch is then used to change the fuel-air mixture and/or to energize a warning signal.
The known apparatus just described has the inherent disadvantage as to be capable of indicating only the complete failure of the sensor if the latter fails to provide a signal of alternating magnitude due to a defect or due to lower than adequate operating temperatures.